www.projectwondeful.org has an auction system for ads. You might want to check it out. You don't get paid per click; you're paid by day per ad. What happens is: advertisers bid on slots for ads on your website, basicly bid money for ad space on your website. Highest offers win and it's win-win.
You might also want to check out Amazon.com and Linkshare.com. Amazon let's you setup something they call an a-store that you can imbed into your site using a frame and LinkShare has you apply for the ability to link to certain stores and you also get credit for any sales made.